MOSCOW -- An improvised memorial near the Kremlin where Russian opposition politician Boris Nemtsov was gunned down last month has been vandalized.

Nemtsov's former colleague Ilya Yashin wrote on Twitter (below) on March 25 that offensive words had been written on pictures of Nemtsov and a plaque calling the bridge where the shooting occurred "Nemtsov's Bridge" had been broken.

Photos of four women and three men standing near the site and holding fragments of the plaque circulated on the Internet late on March 24.

They wore colors that have become a symbol of aggressive patriotism and nationalism.

An unknown group calling itself Liberation Movement SERB claimed responsibility for the vandalism on the Internet.

On March 25, Nemtsov's supporters removed all traces of the vandalism and brought new flowers and portraits to the site.

Nemtsov was fatally shot on a bridge near the Kremlin on February 27.

Several men from the North Caucasus region have been arrested as suspects, but one of the two who have been charged retracted a confession and a rights activist said he may have been tortured.